Hypothesis

Age‑related decline in lysosomal acid sphingomyelinase (ASMase) promotes accumulation of sphingomyelin that traffics to lysosomes, where it nucleates cholesterol crystal formation. These crystals trigger lysosomal membrane permeabilization (LMP), releasing cathepsins B and D into the cytosol. Cytosolic cathepsins cleave the autophagy initiator Beclin‑1 at a conserved site, generating a truncated fragment that fails to support phagophore nucleation. Loss of Beclin‑1 function diminishes autophagosome formation, further reducing lysosomal biogenesis and creating a vicious cycle that amplifies LMP‑driven inflammation and fibrosis in non‑neuronal tissues.

Mechanistic Rationale

1. Sphingomyelin‑cholesterol interplay – Sphingomyelin enriches lipid rafts and promotes cholesterol ordering; loss of ASMase shifts the balance toward intact sphingomyelin, which together with free cholesterol favors crystal nucleation (1).
2. LMP as a crystal‑induced rupture – Cholesterol crystals are known to pierce membranes, causing LMP (1).
3. Cathepsin‑mediated Beclin‑1 cleavage – Cytosolic cathepsin B/D can cleave Beclin‑1 at Asp‑149, a site validated in apoptosis models; the cleaved product lacks the BH3 domain required for VPS34 interaction (4).
4. Feedback to lysosomal biogenesis – Impaired autophagy reduces TFEB activation and lysosome biogenesis, worsening lipid overload and crystal formation (1).

Testable Predictions

• In aged mouse liver and kidney, ASMase activity will be inversely correlated with lysosomal cholesterol crystal burden (measured by filipin staining co‑localized with Lysotracker).
• Genetic or pharmacologic restoration of ASMase will reduce crystal formation, LMP (detected by galectin‑3 puncta), cytosolic cathepsin activity, and Beclin‑1 cleavage, thereby restoring autophagic flux (LC3‑II/I ratio, p62 degradation).
• Expression of a cleavage‑resistant Beclin‑1 mutant (Asp149→Asn) will rescue autophagy and attenuate inflammasome activation (NLRP3, IL‑1β) even when LMP is induced by cholesterol crystals.
• Combined ASMase overexpression with cathepsin B inhibitor (Ca‑074Me) will produce synergistic reduction of fibrosis markers (α‑SMA, collagen I) compared with either intervention alone.

Experimental Approach

• Model: Aged (24‑month) C57BL/6 mice; primary hepatocytes and renal tubular cells isolated for ex‑vivo assays.
• Interventions: AAV8‑ASMase liver‑specific overexpression; small‑molecule ASMase activator (if available); cathepsin B inhibitor Ca‑074Me; CRISPR knock‑in of Beclin‑1 D149N.
• Readouts:
  • Filipin + Lysotracker confocal microscopy for cholesterol crystals.
  • Galectin‑3‑GFP translocation assay for LMP.
  • Western blot for cleaved Beclin‑1 (custom antibody), LC3‑II, p62, cathepsin B/D.
  • ELISA for IL‑1β, NLRP3 inflammasome activation.
  • Hydroxyproline assay and Sirius Red staining for fibrosis.
• Controls: Young (3‑month) mice, AAV‑GFP vector, scrambled siRNA.

If the predictions hold, the data would support a novel axis where sphingomyelin metabolism gates cholesterol crystal‑driven LMP, which through cathepsin‑dependent Beclin‑1 inactivation cripples autophagy, thereby feeding back to worsen lysosomal fragility. Conversely, failure to observe reduced crystals, LMP, or Beclin‑1 cleavage upon ASMase rescue would falsify the hypothesis.